Membrane processes have been widely applied in separation science, and can be applied to a range of separations of species of varying molecular weights in liquid and gas phases (see for example “Membrane Technology and Applications” 2nd Edition, R. W. Baker, John Wiley and Sons Ltd, ISBN 0-470-85445-6).
With particular reference to nanofiltration, such applications have gained attention based on the relatively low operating pressures, high fluxes and low operation and maintenance costs associated therewith. Nanofiltration is a membrane process utilising membranes of molecular weight cut-off in the range of 200-2,000 Daltons. Molecular weight cut-off of a membrane is generally defined as the molecular weight of a molecule that would exhibit a rejection of 90% when subjected to nanofiltration by the membrane. Nanofiltration has been widely applied to filtration of aqueous fluids, but due to a lack of suitable solvent stable membranes, it has not been widely applied to the separation of solutes in organic solvents. This is despite the fact that organic solvent nanofiltration (OSN) has many potential applications in manufacturing industry including solvent exchange, catalyst recovery and recycling, purifications, and concentrations. U.S. Pat. Nos. 5,174,899 5,215,667; 5,288,818; 5,298,669 and 5,395,979 disclose the separation of organometallic compounds and/or metal carbonyls from their solutions in organic media. UK Patent No. GB 2,373,743 describes the application of OSN to solvent exchange; UK Patent No. GB 2,369,311 describes the application of OSN to recycle of phase transfer agents, and; European Patent Application EP1590361 describes the application of OSN to the separation of synthons during oligonucleotide synthesis.
Nanofiltration membranes for aqueous applications are generally fabricated by making composite membranes. Thin film composite membranes may be fabricated via interfacial polymerization (herein also referred to as TP) or by dip-coating [Lu, X.; Bian, X.; Shi, L., “Preparation and characterization of NF composite membrane.” J. Membr. Sci., 210, 3-11, 2002].
In the IP technique, an aqueous solution of a reactive monomer (often a polyamine) is first deposited in the pores of a microporous support membrane, often a polysulfone ultrafiltration membrane. Then, the polysulfone support membrane loaded with the monomer is immersed in a water-immiscible solvent solution containing a reactive monomer, such as diacid chloride in hexane. The two monomers react at the interface of the two immiscible solutions, until a thin film presents a diffusion barrier and the reaction is completed to form a highly cross-linked thin film layer that remains attached to the support membrane. The thin film layer can be from several tens of nanometers to several micrometers thick. The IP technique is well known to those skilled in the art [Petersen, R. J. “Composite reverse osmosis and nanofiltration membranes”. J. Membr. Sci, 83, 81-150, 1993]. The thin film is selective between molecules, and this selective layer can be optimized for solute rejection and solvent flux by controlling the coating conditions and characteristics of the reactive monomers. The microporous support membrane can be selectively chosen for porosity, strength and solvent resistance. A particularly preferred class of thin film materials for nanofiltration are polyamides formed by interfacial polymerization. Examples of such polyamide thin films are found in U.S. Pat. Nos. 5,582,725, 4,876,009, 4,853,122, 4,259,183, 4,529,646, 4,277,344 and 4,039,440, the pertinent disclosures of which are incorporated herein by reference.
U.S. Pat. No. 4,277,344 describes an aromatic polyamide membrane produced by the interfacial polymerization of an aromatic polyamine with at least two primary amine substituents and an acyl halide having at least three acyl halide substituents. Wherein, the aqueous solution contains a monomeric aromatic polyamine reactant and the organic solution contains an amine-reactive polyfunctional acyl halide. The polyamide layer of TFC membranes is typically obtained via an interfacial polymerization between a piperazine or an amine substituted piperidine or cyclohexane, and a polyfunctional acyl halide as described in U.S. Pat. Nos. 4,769,148 and 4,859,384. A way of modifying reverse osmosis (herein also referred to as RO) TFC membranes for nanofiltration is described in U.S. Pat. Nos. 4,765,897; 4,812,270; and 4,824,574. Post-interfacial polymerization treatments have also been used to increase the pore size of TFC RO membranes.
U.S. Pat. No. 5,246,587 describes an aromatic polyamide RO membrane that is made by first coating a porous support material with an aqueous solution containing a polyamine reactant and an amine salt. Examples of suitable polyamine reactants provided include aromatic primary diamines (such as, m-phenylenediamine or p-phenylenediamine or substituted derivatives thereof, wherein the substituent is an alkyl group, an alkoxy group, a hydroxy alkyl group, a hydroxy group or a halogen atom; aromatic secondary diamines (such as, N,N-diphenylethylene diamine), cycloaliphatic primary diamines (such as cyclohexane diamine), cycloaliphatic secondary diamines (such as, piperazine or trimethylene dipiperidine); and xylene diamines (such as m-xylene diamine).
In another method described in U.S. Pat. No. 6,245,234, a TFC polyamide membrane is made by first coating a porous polysulfone support with an aqueous solution containing: 1) a polyfunctional primary or secondary amine; 2) a polyfunctional tertiary amine; and; 3) a polar solvent. The excess aqueous solution is removed and the coated support is then dipped in an organic solvent solution of trimesoyl chloride (TMC) and a mixture of alkanes having from eight to twelve carbon atoms.
Many different types of polymers may be interfacially synthesized using interfacial polymerization. Polymers typically used in interfacial polymerization applications include, but are not limited to, polyamides, polyurea, polypyrrolidines, polyesters, polyurethanes, polysiloxanes, poly(amide imide), poly(ether amide), poly(urea amide) (PUA) [Petersen, R. J. “Composite reverse osmosis and nanofiltration membranes”. J. Membr. Sci, 83, 81-150, 1993]. For example, U.S. Pat. No. 5,290,452 describes the formation of a crosslinked polyester amide TFC membrane produced via interfacial polymerization. The membrane is made by reacting a dianhydride (or its corresponding diacid-diester) with a polyester diol to produce an end-capped prepolymer. The resulting end-capped prepolymer is then reacted with excess thionyl chloride to convert all unreacted anhydride and all carboxylic-acid groups into acid chloride groups. The resulting acid-chloride derivative is dissolved in organic solvent and interfacially reacted with a diamine dissolved in an aqueous phase.
The support membranes generally used for commercial TFC membranes are often polysulfone or polyethersulfone ultrafiltration membranes. These supports have limited stability for organic solvents and, therefore, thin film composites membranes of the prior art which are fabricated with such supports cannot be effectively utilized for all organic solvent nanofiltration applications.
Although interfacially polymerized TFC membranes of the prior art have been specifically designed to separate aqueous feed streams down to a molecular level, they can be applied in certain organic solvents as well [Koseoglu, S. S., Lawhon, J. T. & Lusas, E. W. “Membrane processing of crude vegetable oils pilot plant scale removal of solvent from oil miscellas”, J. Am. Oil Chem. Soc. 67, 315-322, 1990, U.S. Pat. No. 5,274,047]. Their effectiveness depends on the specific molecular structure of the thin film layer and the stability of the support membrane. U.S. Pat. No. 5,173,191, suggests nylon, cellulose, polyester, Teflon and polypropylene as organic solvent resistant supports. U.S. Pat. No. 6,986,844 proposes the use of crosslinked polybenzimidazole for making suitable support membranes for TFC. TFC membranes comprising a thin film synthesized from piperazine/m-phenylenediamine and trimesoyl chloride on a PAN support membrane performed well in methanol, ethanol and acetone, less well in i-propanol and MEK, and gave no flux in hexane [Kim, I.-C., Jegal, J. & Lee, K.-H. “Effect of aqueous and organic solutions on the performance of polyamide thin-film-composite nanofiltration membranes.” Journal of Polymer Science Part B: Polymer Physics 40, 2151-2163, 2002].
US 2008/0197070 describes the formation of thin film composite membranes on polyolefin (e.g. polypropylene) supports prepared by interfacial polymerization. These membranes performed well in water, ethanol and methanol.
Non-reactive polydimethylsiloxane (PDMS) has been added during the interfacial polymerization reaction using polyacrylonitrile (PAN) as the support membrane [Kim, I. C. & Lee, K. H. “Preparation of interfacially synthesized and silicone-coated composite polyamide nanofiltration membranes with high performance.” Ind. Eng. Chem. Res. 41, 5523-5528, 2002, U.S. Pat. No. 6,887,380, U.S. Pat. Applic No. 0098274 2003]. The resulting silicone-blended PA membrane showed high hexane permeabilities.
TFC membranes have also been applied for filtration in apolar solvents. A method for the separation of lube oil from organic solvents (e.g. furfural, MEK/toluene, etc.) with a TFC membrane using poly(ethylene imine) and a diisocyanate on a solvent resistant nylon 6,6 support has been described in U.S. Pat. No. 517,391.
In interfacially polymerized composite membranes, both the surface chemistry and the morphology of the support membrane play a crucial role in determining the overall composite membrane performance. Membrane performance can be enhanced through modification of the membrane surface [D. S. Wavhal, E. R. Fisher, “Membrane surface modification by plasma-induced polymerization of acrylamide for improved surface properties and reduced protein fouling”, Langmuir 19, 79, 2003]. Thus, different procedures have been carried out to chemically modify the membrane surface and modify its properties. These procedures may increase the hydrophilicity, improve selectivity and flux, adjust transport properties, and enhance resistance to fouling and chlorine. Many methods have been reported for membrane surface modification such as grafting, coating [U.S. Pat. Nos. 5,234,598, 5,358,745, 6,837,381] and blending of hydrophilic/-phobic surface modifying macromolecules (SMMs) [B. J. Abu Tarboush, D. Rana, T. Matsuura, H. A. Arafat, R. M. Narbaitz, “Preparation of thin-film-composite polyamide membranes for desalination using novel hydrophilic surface modifying macromolecules”, J. Membr. Sci. 325, 166, 2008].
In order to improve the performance of TFC membranes, different constituents have been added to the amine and/or acyl halide solutions. For example, U.S. Pat. No. 4,950,404, describes a method for increasing flux of a TFC membrane by adding a polar aprotic solvent and an optional acid acceptor to the aqueous amine solution prior to the interfacial polymerization reaction. In a similar way, U.S. Pat. Nos. 5,989,426; 6,024,873; 5,843,351; 5,614,099; 5,733,602 and 5,576,057 describe the addition of selected alcohols, ketones, ethers, esters, halogenated hydrocarbons, nitrogen-containing compounds and sulfur-containing compounds to the aqueous amine solution and/or organic acid halide solution prior to the interfacial polymerization reaction.
It has been claimed that soaking freshly prepared TFC membranes in solutions containing various organic species, including glycerol, sodium lauryl sulfate, and the salt of triethylamine with camphorsulfonic acid can increase the water flux in RO applications by 30-70% [3]. As described in U.S. Pat. Nos. 5,234,598 and 5,358,745, TFC membrane physical properties (abrasion resistance), and flux stability can also be improved by applying an aqueous solution composed of poly(vinyl alcohol) (PVA) and a buffer solution as a post formation step during membrane preparation. Adding alcohols, ethers, sulfur-containing compounds, monohydric aromatic compounds and more specifically dimethyl sulfoxide (DMSO) in the aqueous phase can produce TFC membranes with an excellent performance [S.-Y. Kwak, S. G. Jung, S. H. Kim, “Structure-motion-performance relationship of flux-enhanced reverse osmosis (RO) membranes composed of aromatic polyamide thin films”, Environ. Sci. Technol. 35, 4334, 2001; U.S. Pat. Nos. 5,576,057; 5,614,099]. After addition of DMSO to the interfacial polymerization system, TFC membranes with water flux five times greater than the normal TFC water flux with a small loss in rejection were obtained [S. H. Kim, S.-Y. Kwak, T. Suzuki, “Positron annihilation spectroscopic evidence to demonstrate the flux-enhancement mechanism in morphology-controlled thin-film-composite (TFC) membrane”, Environ. Sci. Technol. 39, 1764, 2005].
However, in these prior art TFC membranes the use of a polysulfone support membrane limits the potential for additives to either aqueous amine solution or organic acid halide solution.
Several methods for improving the membrane performance post-formation are also known. For example, U.S. Pat. No. 5,876,602 describes treating the TFC membrane with an aqueous chlorinating agent to improve flux, lower salt passage, and/or increase membrane stability to bases. U.S. Pat. No. 5,755,965 discloses a process wherein the surface of the TFC membrane is treated with ammonia or selected amines, e.g., 1,6, hexane diamine, cyclohexylamine and butylamine. U.S. Pat. No. 4,765,879 describes the post treatment of a membrane with a strong mineral acid followed by treatment with a rejection enhancing agent.
A method of chemical treatment is claimed to be able to cause a simultaneous improvement of water flux and salt rejection of thin-film composite (TFC) membranes for reverse osmosis [Debabrata Mukherjee, Ashish Kulkarni, William N. Gill, “Chemical treatment for improved performance of reverse osmosis membranes”, Desalination 104, 239-249, 1996]. Hydrophilization by treating the membrane surface with water soluble solvent (acids, alcohols, and mixtures of acids, alcohols and water) is a known surface modification technique. This method increases the flux without changing the chemical structure [Kulkarni, D. Mukherjee, W. N. Gill, “Flux enhancement by hydrophilization of thin film composite reverse osmosis membranes”, J. Membr. Sci. 114, 39, 1996]. Using a mixture of acid and alcohol in water for the surface treatment can improve the surface properties, since acid and alcohol in water cause partial hydrolysis and skin modification, which produces a membrane with a higher flux and a higher rejection. It was suggested that the presence of hydrogen bonding on the membrane surface encourages the acid and water to react on these sites producing more charges [D. Mukherjee, A. Kulkarni, W. N. Gill, “Flux enhancement of reverse osmosis membranes by chemical surface modification”, J. Membr. Sci. 97, 231, 1994]. Kulkarni et al. hydrophilized a TFC-RO membrane by using ethanol, 2-propanol, hydrofluoric acid and hydrochloric acid. They found that there was an increase in hydrophilicity, which led to a remarkable increase in water flux with no loss in rejection.
A hydrophilic, charged TFC can be achieved by using radical grafting of two monomers, methacrylic acid and poly(ethylene glycol) methacrylate onto a commercial PA-TFC-RO membrane [S. Belfer, Y. Purinson, R. Fainshtein, Y. Radchenko, O. Kedem, “Surface modification of commercial composite polyamide reverse osmosis membranes”, J. Membr. Sci. 139, 175, 1998]. It was found that the use of amine containing ethylene glycol blocks enhanced the performance of the membrane, and highly improved membrane water permeability by increasing hydrophilicity [M. Sforca, S. P. Nunes, K.-V. Peinemann, “Composite nanofiltration membranes prepared by in-situ polycondensation of amines in a poly(ethylene oxide-b-amide) layer”, J. Membr. Sci. 135, 179, 1997]. Poly(ethylene glycol) (PEG) and its derivatives have been used for surface modification. TFC membrane resistance to fouling could be improved by grafting PEG chains onto the TFC-RO membranes [1, 2].
PEG has also been used to improve the TFC membrane formation [Shih-Hsiung Chen, Dong-Jang Chang, Rey-May Liou, Ching-Shan Hsu, Shiow-Shyung Lin, “Preparation and Separation Properties of Polyamide Nanofiltration Membrane”, J Appl Polym Sci, 83, 1112-1118, 2002]. Because of the poor hydrophilicity of the polysulfone support membrane, poly(ethylene glycol) (PEG) was added to the aqueous solution as a wetting agent. The effect of PEG concentration on the resulting membrane performance was also studied.
It has been reported that PEG is frequently used as an additive in the polymer solution to influence the membrane structure during phase inversion [Y. Liu, G. H. Koops, H. Strathmann, “Characterization of morphology controlled polyethersulfone hollow fiber membranes by the addition of polyethylene glycol to the dope and bore liquid solution”, J. Membr. Sci. 223, 187, 2003]. The role of these additives is to create a spongy membrane structure by prevention of macrovoid formation and enhance pore formation during phase inversion. Other frequently used additives are: glycerol, alcohols, dialcohols, water, polyethylene oxide (PEO), LiCl and ZnCl2. US patent Nos. 2008/0312349 A and 2008/207822 A also describe the use of PEG in the polymeric dope solution during preparation of microporous support membranes.
Prior art TFC membranes are not claimed to be suited for filtrations in harsh solvents (e.g. THF, DMF). Thus, current and emerging applications, using non-aqueous media in pressure-driven membrane processes, present a need for production of membranes that exhibit greater stability. The membrane products and membrane-related methods of the present invention advantageously address and/or overcome the obstacles, limitations and problems associated with current membrane technologies and effectively address membrane-related needs that are noted herein.